A common method of constructing walls and ceilings includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as “wallboard” or “drywall.” Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional walls made from wet plaster methods, is desirable because the installation of wallboard is ordinarily less costly and less cumbersome when compared with the installation of conventional plaster walls.
Walls and ceilings made with gypsum wallboard panels typically are constructed by securing, e.g., with nails or screws, the wallboard panels to structural members, such as vertically- and horizontally-oriented pieces of steel or wood often referred to as “studs.” When forming a wall from wallboard panels there will generally be “joints” between adjacent wallboard panels because wallboard is typically supplied in standard-sized sheets. In most wallboard construction, the joints are cosmetically treated with reinforcing tape and an adhesive material called “joint compound” so that the wall will have a smooth finish similar to that obtained with conventional plaster walls.
Generally, wallboard is produced by enclosing a core of an aqueous slurry containing calcined gypsum and other materials between two large sheets of wallboard cover paper. Various types of cover paper are known in the art. After the gypsum slurry has set (i.e., reacted with the water present in the aqueous slurry) and dried, the formed sheet is cut into standard sizes. Methods for the production of gypsum wallboard generally are described, for example, by Michelsen, T. “Building Materials (Survey),” Kirk-Othmer Encyclopedia of Chemical Technology, (1992 4th ed.), vol. 4, pp. 618-619, the disclosure of which is hereby incorporated herein by reference.
A major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as “calcined gypsum,” “stucco,” or “plaster of Paris.” Stucco has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH.
Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step in the manufacturer of stucco includes passing crude gypsum rock through a rotary kiln to remove any free moisture (i.e., water which is not chemically bound) present in the rock.
The dried rock is then passed through a roller mill (or impact mill type of pulverizer), wherein the rock is ground or comminuted to a desired fineness. The dried, fine-ground gypsum can be referred to as “land plaster.” The land plaster is used as feed in calcination processes for conversion to stucco.
The calcination (or dehydration) step in the manufacture of stucco is performed by heating the land plaster to liberate a portion of the chemically bound water molecules. Calcination of stucco can generally be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor:CaSO4•2H2O+heat→CaSO4•½H2O+1½H2O. 
This calcination process step is performed in a “calciner,” of which there are several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e., land plaster) is the “stable” form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. The setting reaction is a reversal of the above-described chemical reaction that occurs during the calcination step. Accordingly, the setting reaction proceeds according to the following chemical equation, which shows that calcium sulfate hemihydrate is rehydrated to its dihydrate state upon the addition of sufficient water:CaSO4•½H2O+1½H2O→CaSO4•2H2O+heat. 
The actual time required to complete the setting reaction generally depends upon the type of calciner and the type of gypsum rock that is used to produce the gypsum, and can be controlled to some extent by using additives such as, for example, retarders, set accelerators, and/or stabilizers. Generally, the time required for rehydration can be as little as about two minutes to as long as about eight hours, depending on the quantity of retarders, set accelerators, and/or stabilizers present.
Gypsum wallboard is generally manufactured utilizing commercial processes that are capable of operation under continuous, high-speed conditions. A conventional process for manufacturing the core composition of gypsum wallboard initially includes the premixing of dry ingredients in a high-speed mixing apparatus. The dry ingredients can include calcium sulfate hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch).
The dry ingredients are typically mixed together with a “wet” (aqueous) portion of the core composition in a pin mixer apparatus. The wet portion can include a first component, commonly referred to as a “paper pulp solution,” that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and a set retarder. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of strengthening agents, foaming agents, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise an aqueous gypsum slurry that eventually forms a gypsum wallboard core.
After the aqueous gypsum slurry is prepared, the slurry and other desired ingredients are continuously deposited to form a gypsum wallboard core (hereinafter “wallboard core” or “core”) slurry between two continuously-supplied moving sheets of cover paper. The two cover sheets are typically a pre-folded face paper and a backing paper. As the slurry is deposited onto the face paper, the backing paper is brought down atop the deposited core slurry and bonded to the prefolded edges of the face paper.
The whole assembly is sized for thickness utilizing a roller bar or forming plate. The deposited core slurry is then allowed to set between the two cover sheets, thereby forming a board. The continuously-produced board is cut into panels of a desired length, which are vertically-stacked, and then passed through a drying kiln wherein excess water is removed from the board to form a strong, rigid, fire-resistant building material.
The cover sheets used in the process typically are multi-ply paper manufactured from re-pulped newspapers. The face paper has an unsized inner ply which contacts the core slurry such that gypsum crystals can grow up to (or into) the inner ply. This gypsum crystal-paper interaction is the principal form of bonding between the core slurry and the cover sheet. The middle plies are sized and an outer ply is more heavily sized and treated to control absorption of paints and scalers. The backing paper is also a similarly constructed multi-ply sheet. Both cover sheets must have sufficient permeability to allow for water vapor to pass through during the downstream board drying step(s).
Standardized sheets (or panels) of wallboard typically are about four feet (about 1.22 meters) wide and about 8 feet to about 16 feet (about 2.4 meters to about 4.9 meters) in length. Sheets typically are available in thicknesses varying in a range of about ¼ inch to about one inch (about 0.6 centimeters to about 2.6 centimeters).
In order to provide satisfactory strength, commercially-available gypsum wallboard generally requires a density of about 1650 pounds to about 1700 pounds (about 748 kilograms to about 772 kilograms) per thousand square feet (lbs/MSF) of one-half inch board. Because heavy or high-density gypsum wallboards are more costly and difficult to manufacture, transport, store, and manually install at job sites when compared with lighter or low-density boards, various attempts have been made to reduce board weight and density without sacrificing board strength.
Often, however, where wallboard is formulated to have a density less than about 1650 lbs/MSF of one-half inch board, the resulting strength is unacceptable for commercial use.
While it is possible to formulate lighter and less dense wallboard, for example, through the inclusion of lightweight fillers and foams into a gypsum slurry, many of the lighter and less dense wallboard products are of a quality ill-suited for commercial use. It has been suggested that reduced density wallboard of acceptable strength can be obtained by incorporating lightweight thermoplastic particles such as, for example, pre-expanded polystyrene or polyethylene latex polymer particles, into the gypsum slurry. However, this proposed solution is deficient because latex polymers typically form aggregates in a high ionic strength environment, such as, for example, that of a gypsum slurry. Consequently, latex polymer particles typically are not dispersed uniformly throughout the gypsum product. Such a non-uniform composition is very undesirable because it can result in local areas of weakness in the final gypsum product.
U.S. Pat. No. 6,171,388 to Jobbins suggests that the non-uniform distribution of latex polymer particles can be overcome by adding an excess of nonionic surfactant(s) along with one or more latex polymers to a gypsum slurry. The inventor explains that the latex particles are better distributed throughout the slurry, that the resulting material has an improved strength to weight ratio, and that the slurry viscosity is decreased because of the added surfactant(s). Accordingly, by increasing the amount of nonionic surfactants in the gypsum slurry, the stability of the latex particles against a high ionic environment is improved.
However, while it may be possible to formulate gypsum slurries to include additional surfactants in order to confer substantial stability to light-weight polymeric particles against a high ionic strength environment, gypsum slurries already contain a significant amount of surfactants and/or foaming agents (e.g., to facilitate the introduction of air bubbles into the gypsum slurry in order to reduce gypsum board weight). The various surfactants and/or foaming agents present in the gypsum slurry may interact with one another, thereby reducing their effectiveness, both as foaming agents and in conveying stability to the latex polymer particles against the high ionic strength environment of a gypsum slurry. Accordingly, it is desirable to avoid introducing an excess of additional surfactants into the gypsum slurry.
In view of the foregoing, it would be desirable to produce high-strength gypsum wallboard having weights and densities generally equal to or slightly less than those produced by conventional methods. Furthermore, such reduced weight and density boards should have a composition which is substantially uniform throughout the gypsum product, and should meet industry standards, having strengths similar to, or greater than, conventional wallboard. Moreover, such wallboard also should be able to be manufactured using high-speed manufacturing apparatus and not suffer from other negative side-effects. For example, such high-strength wallboard should be able to set and dry within a reasonable period of time.